1. Field of the Invention
The present invention relates to an optical characteristic measurement apparatus for recognizing the best image plane position from the defocus characteristic of an optical system to be tested.
2. Description of the Related Art
In every optical field irrespective of a specific field to which an optical system is applied, an image condition is observed while moving a camera in the optical axis direction in proximity to a target image plane of an optical system to be tested, in order to measure the focusing position and the focal depth performance of the optical system to be tested. The measurement allows recognition of the difference between the best image plane position and the target image plane position of the optical system to be tested, and enables the difference to be reduced by correcting the shapes of components and adjusting the positions according to the value, as necessary. Instead thereof or in addition thereto, the measurement enables the appropriateness of depth allowance amount for variation in focal position caused by variation in temperature and a mechanical error caused by assembling into a higher unit or a product to be determined.
For instance, Japanese Patent Application Laid-Open No. 2007-163227 discloses an apparatus for measuring the performance of a laser scanning optical system. This apparatus has a configuration that can arbitrarily, automatically change a relative separation between a jig for measuring an optical characteristic on which an optical system to be tested is mounted and a beam measurement unit configured by a CCD camera. This document also discloses testing of the depth allowance amount of a scanning lens by measuring a depth characteristic, which is referred to as depth curve.
Japanese Patent Application Laid-Open No. H07-120691 describes variation in a beam spot diameter with respect to a defocus amount, which is a distance from a target image plane position, and discloses that asymmetric characteristics are exhibited according to positive or negative variation in distance.
In the case of evaluating the defocus characteristic of the optical system to be tested while varying one of the distance between an object and the optical system to be tested and the distance between the optical system to be tested and an evaluation apparatus, such as a screen and a camera, a following problem is caused. This problem is that, if the defocus amount is large with respect to the distance between the pupil of the optical system to be tested and an evaluation reference plane, it is difficult to determine the true best image plane position. The center of a region smaller than a region with an allowable spot diameter is typically adopted as the best image plane position. Unfortunately, the best image plane position varies with variation in value of the allowable spot diameter.